Characterization of digital radio signals

ABSTRACT

A method and apparatus for visually characterizing modulated carrier signals, particularly digitally modulated microwave signals, which may be subjected to multipath fading distortion. The method involves sampling the amplitude dispersion of the received signal during a test period, counting the number of times the sampled values fall within each of a number of value ranges and displaying the count per range against range so that counts corresponding to positive and negative amplitude dispersion values are shown in a symmetrical fashion. The shape of the resultant display provides a useful characterization of the signal.

This is a divisional of application Ser. No. 829,150 filed Feb. 24,1986, now U.S. Pat. No. 4,766,600.

This invention relates to characterisation of modulated electricalsignals, particularly but not exclusively for the purposes of evaluationof performance of digital radio data signal systems or of components foruse in such systems.

It is well known that radio signals generated as a modulated carrier maybe received in down-graded form at the receiver of a transmission systemfor a variety of reasons. A particular form of signal degradation knownas multipath interference occurs wherein differently directed componentsof the transmitted signal both reach the receiver after travelling alongpaths of different lengths. If the signal components arrive in-phase,constructive interference will arise, and if the signal componentsarrive out of phase, destructive interference will arise. For givensignal paths for the two signal components, a multi-frequency signalsuch as the digitally modulated carrier mentioned will be affecteddifferently at different frequencies. At some frequencies, constructiveinterference will occur whilst at others destructive interference willoccur. In the former case an increase in signal strength will beapparent, and in the latter case, a decrease will be apparent. Theresultant alternate nodes and antinodes in the plot of signal strengthagainst signal frequency, at which constructive and destructiveinterference arise, may give rise to signal distortion which rendersdemodulation of the received signal difficult. This phenomenon arisesbecause of existence of particular atmospheric conditions, and variationof these conditions may result in shifting of the transmitted signalnodes and antinodes back and forth along the bandwidth of thetransmitted signal so that the signal, as received, is caused to vary inan unpredictable fashion, further increasing the difficulty ofdemodulating. Various strategies including use of various types ofcompensating circuitry are employed in receivers for the purpose ofminimising errors in these circumstances.

In order to evaluate the performance of circuitry for reducingdemodulation errors in, say, a receiver it is customary to apply to thereceiver a simulated multipath interference signal Circuitry is employedpermitting generation, from a single input signal, of a pair of phaseshifted component signals the relative magnitudes and phase shift and/ordelay of which are variable. These component signals may be in the formof digitally modulated carriers and they are combined and fed to thereceiver The output of the receiver is monitored and the relative gainsof the two component signals varied, for each of a number of phaseshifts, so that various combinations of phase shift and relative gainare determined, for which combinations the error ratio in thedemodulated signal just reaches some predetermined error ratio. Fromthis data a graph is plotted of relative signal proportion, as betweenthe out of phase signals, against relative phase shift or against thefrequency of an anti-node or "notch" in the frequency spectrum of thecombined signal, since the phase shift is directly related to this notchfrequency This graph will be representative of the notch depth and notchposition that will produce a particular bit error ratio in the outputdata signal.

A graph so obtained thus represents a characterisation of the combinedtest signals on the basis that notching at particular positions in thefrequency bandwidth thereof will give rise to an error ratio equal tothe predetermined ratio in the demodulated signal.

The preparation of these graphs is laborious, making testing slow.Furthermore, the resultant graph is obtained on the basis of staticrelationships between the component signals, whereon in a practicalenvironment the relationship tends to change in a random fashion.Circuitry which performs well under static conditions in reducingdemodulation errors may not be able to perform adequately under changingconditions so that the described method provides only an indirect guideto in-service performance.

An object of the invention is to provide an improved method ofcharacterising electrical signals.

Thus, in one aspect, the invention provides a method of characterising amodulated carrier signal comprising:

(a) repetitively sampling at time spaced intervals the in band amplitudedispersion of the signal;

(b) accumulating first counts of numbers of occurrences of respective inband dispersion values over a range of said values;

(c) monitoring a parameter indicative of signal quality of a signalobtained by demodulating said modulated carrier signal;

(d) accumulating second counts of numbers cf occurrences of respectivein band amplitude dispersions of values within said range, and whichlast mentioned in band amplitude dispersions, at least substantiallycoincide with the value of said parameter crossing a predeterminedlevel; and

(e) dividing the second counts for each said in band amplitudedispersion value by the first count therefor, to obtain respectivedivided counts each representative of the probability that, at therespective in band amplitude dispersion value, the value of saidparameter will cross said predetermined level.

Where the method is used for characterising a digitally modulatedcarrier signal, the monitored parameter may be the error ratio in thesignal obtained by demodulating the digitally modulated carrier signal.Where the method is used for characterising an analogue modulatedcarrier signal the monitored parameter may be the signal-noise ratio ofthe signal obtained by demodulating the analogue modulated carriersignal.

The first counts are representative of a first histogram of frequency ofoccurrence of the various in band amplitude dispersion values, and themethod may comprise generating this first histogram.

The second counts are representative of a second histogram of frequencyof occurrence of the various in band amplitude dispersion values whichcoincide with occurrence of the aforementioned predetermined level beingcrossed. The method may comprise generating this second histogram.

The divided counts are representative of a third histogram ofprobabilities that, at various in band amplitude dispersion values,value of the measured parameter, such as the error ratio, will crosssaid predetermined level. The method may comprise generating this thirdhistogram.

The method of the invention may be applied where said modulated signalis a directly received radio signal or it may be applied to signalsderived therefrom such as the intermediate frequency signal in asuperhetrodyne or like receiver.

The in band amplitude dispersion samples may be generated by a procedureof sampling, at substantially corresponding times, signal magnitudes attwo different frequencies within the bandwidth of said modulated carriersignal, such as at frequencies spaced by equal frequency differencesfrom the carrier frequency, and being located towards opposite ends ofthe usable bandwidths, and subtracting the sampled signal magnitudes indecibels at one said frequency from sampled signal magnitudes indecibels at said other frequency taken at corresponding times.

The dispersions may be assigned as negative or positive depending uponwhether the magnitudes of sampled signals associated with a particularone of said two different frequencies are greater or less than thecorresponding sampled signals associated with the other of said twofrequencies. More complex methods of determining the in band amplitudedispersion, as practised in the art, may be employed, such as thoseinvolving alegraic combination of more than two samples at respectivedifferent frequencies.

The modulated carrier signal may be distorted by mixing of signalcomponents of the same frequency where the phase and amplituderelationship between the components is continually varied, such ascyclically and substantially continuously.

The range of variations of phase and amplitude between the signalcomponents is preferably selected so as to cause a notch in thefrequency spectrum of the combined signal to move back and forth acrossthe full bandwidth of that signal. In this case, the carrier signal maybe randomly or pseudo randomly modulated, such as by phase and/oramplitude modulation.

The invention also provides apparatus for characterising a modulatedcarrier signal comprising;

(a) means for generating time spaced samples of in band amplitudedispersion of the signal;

(b) means for accumulating first counts of numbers of occurrences ofrespective in band dispersion values, over a range of said values;

(c) means for monitoring a parameter indicative of signal quality of asignal obtained by demodulating said modulated carrier signal;

(d) means for accumulating second counts of numbers of occurrences ofrespective in band amplitude dispersions of values within said range,and which last mentioned in band amplitude dispersions at leastsubstantially coincide with the value of said parameter crossing apredetermined level; and

(e) means for dividing the second counts for each said in band amplitudedispersion value by the first count therefor, to obtain respectivedivided counts each representative of the probability that, at therespective in band amplitude dispersion value, the value of saidparameter will cross said predetermined level.

This means for generating samples of in band amplitude dispersion maycomprise two receiver devices each in use receiving said modulatedcarrier signal and responsive to separate frequency signal componentswithin the bandwidth of said modulated carrier signal, together withmeans for subtracting the output of one said receiver in decibels fromthe output of the other in decibels.

The invention may be practised by accumulating counts of numbers ofoccurrences of respective in band amplitude dispersions of values withinsaid range, and which in band amplitude dispersions at leastsubstantially coincide with said level crossing said predetermined levelby exceeding that level. However, the invention may also be practised byaccumulating counts of numbers of occurrences of respective in bandamplitude dispersions of values within said range and which in bandamplitude dispersions at least substantially coincide with said levelfalling below said predetermined level.

The invention is further described, by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 is a diagram illustrating the mechanism for production formultipath interference in radio signals;

FIG. 2 is a graph illustrating variation in effect of with frequency;

FIGS. 3(a), 3(b) and 3(c) are diagrams illustrating the effect ofmultipath interference on signal strength over the bandwidth of a radiosignal;

FIG. 4 illustrates a prior art simulator for use in testing of radioreceivers;

FIG. 5 shows a form of graphical result obtained by use of the simulatorof FIG. 4;

FIG. 6 is block diagram of a dynamic dispersion receiver constructed inaccordance with this invention;

FIG. 7 is a block diagram of a system in accordance with the inventionfor dynamic dispersion testing of digital radio equipment;

FIG. 8 is a block diagram of a system for dynamic dispersion testing ofdigital radio equipment, in accordance this invention;

FIG. 9 is a block diagram of a system for field diagnosis of digitalradio systems in accordance with the present invention;

FIG. 10 is a histogram obtained by use of the system of FIG. 7, showingnumbers of occurrences of in band amplitude dispersions of variousvalues;

FIG. 11 is a histogram obtained by use of the system of FIG. 7, showingnumbers of occurrences of in band amplitude dispersions of variousvalues which are substantially coincident with occurrence of greaterthan a predetermined error ratio in demodulated signals;

FIG. 12 is a histogram obtained by use of the system of FIG. 7, showingprobability that the error ratio in demodulated transmissions willexceed a predetermined error ratio at particular in band amplitudedispersions;

FIG. 13 is histogram like that shown in FIG. 12 but obtained by use of ademodulator different to that used to obtain the histogram of FIG. 12;

FIG. 14 is a block diagram of two narrow band receivers incorporatedinto the amplitude dispersion receiver of FIG. 6;

FIG. 15 part of a detailed circuit diagram of one of the narrow bandreceivers shown in FIG. 14;

FIG. 16 detailed circuit diagram of a local oscillator in FIG. 14;

FIG. 17 detailed circuit diagram of a branching amplifier shown in FIG.19; and

FIGS. 18 and 19 join on the line X--X shown in each to form a flowdiagram for data manipulation in accordance with the invention.

In FIG. 1, a radio transmitter 30 is shown arranged for radiation ofdigitally modulated radio signals from a transmitter antenna 32 to areceiving antenna 34 coupled to a receiver 36 for the radio signals. Ifcomponent radio signals radiated from antenna 32 do not travel on asingle path, such as that denoted by the arrows 38, from the antenna 32to the antenna 34, but also traverse a different, longer, path such asillustrated by the arrows 40, the antenna 34 will receive a combinedsignal the magnitude of which will vary considerably with frequencydepending upon whether, at any particular chosen frequency, constructiveor destructive interference occurs. The effect of this multipathinterference is shown in FIG. 2 where the amplitude of received signalis shown by graph 42 as exhibiting a cyclic change with frequency,exhibiting nodes 42a of relatively high signal strength alternating withantinodes or "notches" 42b of substantially reduced signal strength.

FIG. 3(a) shows a typical frequency spectrum of a digitally modulatedradio signal. This spectrum is representative of a spectrum which mightappertain to, say, a 16QAM digital data transmission system signal Theamplitude of the spectrum envelope is constant across the usablebandwidth of the signal extending to either side of the carrierfrequency f_(c). Circuitry for correctly demodulating the signal isreliant on the Presumption that this graph is so configured. However,FIGS. 3(b) and 3(c) illustrate possible distortions of the spectrumenvelope occurring, on the one hand, if the bandwidth of the transmittedradio signal falls adjacent to and at one side of a notch 42b, asrepresented by B₂ in FIG. 2, or, adjacent to and at the other sidethereof, as shown by B₃ in FIG. 2. It will be seen that, in either case,the envelope of the frequency spectrum is distorted from theconfiguration of FIG. 3(a) to have a rising signal magnitude withincrease in frequency or falling magnitude with decrease in frequency.If there is a pronounced distortion of this kind, accurate demodulationof signal information is made difficult and substantial errors may occurin demodulation. The extent to which this phenomenum occurs is dependentupon the relative positions of any notches 42b relative to the bandwidthof the transmitted signal and the relative strengths of the multipathsignal components, the effect being worsened with increased relativestrength of the non-directly arriving signal components. In a practicalenvironment, both the positions and magnitudes of the notches may varywith variations in atmospheric conditions so that the effects on signaldegradation at the receiver are constantly changing. Circuitry may beincorporated into the receiver to compensate for these variations and,in order to test the performance of such circuitry, test setups ofvarious kinds have been used. FIG. 4 shows an exemplary form ofapparatus for this purpose.

In FIG. 4, a signal simulator is shown coupled to the output of agenerator 50 of digitally modulated signal. The signal may be generatedat radio frequency, or some lower frequency such as the intermediatefrequency of a receiver to be tested may be employed. Signal branches51, 53 are coupled to the output of the generator 50 by a suitablecoupling device 52. Signal component in branch 51 is passed directlythrough an attenuator 57 whilst that in path 53 passes firstly through afixed delay device 54, thence through a variable attenuator 56, and thenthrough a variable phase device 58 which can be adjusted to providevariable phase displacement. Signal components leaving attenuator 57 anddevice 58 are combined by a suitable coupling device 60 and are thencefed via a variable attenuator 62 to a receiver 64 under test. Wheregenerator 50 generates radio frequency signals, the output fromattenuator 62 is applied directly to the receiver input but if theintermediate frequency is generated, the signal is fed to the receiver64 at a location past the local oscillator.

The attenuators and device 58 are manipulated, whilst monitoring thedemodulated output from the receiver 64, to produce a graph 66 of thekind shown in FIG. 5. It will be appreciated that the combined signalcomponents, after travelling through the branches 51, 53 and combiningat coupler 60 will interfere in a fashion analogous to the fashiondescribed in relation to signals travelling from the transmitter antenna32 to the receiver antenna 34 via the two paths shown in FIG. 1.Consequently, by varying the phase difference provided by device 58, anotch analogous to one of the notches 42b described in relation to FIG.2 can be positioned at any desired location across the bandwidth of thesignal, so distorting the envelope of the frequency spectrum such as infashions analogous to that shown in FIG. 3(b) or 3(c), to an extentwhich is dependent on the relative strengths of the signal componentsfrom the two branches 51, 53. The graph 66 shown in FIG. 5, called a"static notch signature" is made by monitoring the bit error ratio atthe output of the amplifier 64 and adjusting the attenuator 56 for eachof a number of different phase differences provided by device 58 until adesired reference bit error ratio exists at the output. Since relativephase difference provided by the device 58 determines the position ofnotch 42b in the frequency spectrum of the signal, that phase differencecan be equated directly to a frequency within the bandwidth of thesignal, so that the horizontal axis of the graph of FIG. 5 may belabelled in terms of the phase difference or, as shown, in terms offrequency. The vertical axis of the graph is shown as representing therelative strengths of signal components from branches 51, 53 in themixed signal applied to the receiver. These relative strengths aredetermined by the attenuation ratio as between the attenuators 56, 57.The point labelled "O" on the vertical axis of the graph of FIG. 5illustrates a point at which the attenuator 56 is set to provide nosignal flow through branch 53. The Point labelled "1" represents a pointat which the signal strengths through the two branches are equal. Thestatic notch signature 66 is representative of the notch depth and notchposition that will produce a particular bit error ratio in the outputdata signal. The effect of various forms of compensatory circuitry i thereceiver 64 can be assessed by determining whether the signature 66 isaffected in a fashion tending to make its encompassed size less orgreater. For example, a typical variation to the form of the signature66 occurring through use of known compensatory circuitry on a typicalreceiver is shown by line 66a in FIG. 5.

As mentioned previously, the plotting of the signature 66 shown in FIG.5 is time consuming and is generally effected manually, whilst, in anyevent, it fails to represent performance of the receiver 64 at otherthan static conditions, where the ratios of attenuations time delays andphase shifts between the combined signal portions varies continually.

Turning now to FIG. 6, there is shown a dynamic dispersion receiver 120constructed in accordance with the invention. This comprises tworeceivers 70, 80 arranged to receive on a line 98 the intermediatefrequency signal from a receiver under test. These are coupled viaanalog/digital converters 72, 82 to a dataprocessor system 84 whichadditionally receives an input, on a line 86, from an error detector 85monitoring the output from the receiver under test and deliveringdigital signals to the system 84 indicative of errors (and theirmagnitudes) in the decoded output from the receiver. The system 84includes a processor 89, digital/analog converters 88, 90, and afunction control device 94 which controls processor 89. Output from theprocessor 89 is provided on the digital/analog converters 88, 90 whichcouple to a suitable graphical representation device included in system84 and comprising for example a cathode ray oscilloscope or the X/Yplotter 87 shown. Processor 89 is controlled in accordance withconventional practice under the function control device 94. Thereceivers 70 and 80 have narrow bandwidths and are tuned to receivesignals in these narrow bandwidths and at frequencies located towardsrespectively opposite ends of the frequency spectrum envelope of thesignal on the inputs thereto. Thus, the receivers may have a bandwidthof 0.5 MHz. The receivers are arranged to provide an output withresolution sufficient to enable resolution of a substantial number ofamplitude or signal level steps in signal applied thereto. It has beenfound satisfactory to provide for a resolution of 0.5 dB of the signalstrength at the particular reference frequencies. These signalmagnitudes are repetitively sampled, such as at a rate of ten samplesper second and the sample magnitudes converted to digital form by theconverters 72, 82 and fed to the dataprocessor system 84. The signal online 86 applied to the data processor system 84 is in the form of asignal which is pulsed once for every error. A counter of these errorsis incorporated in the processor 89, this counter being repetitivelyup-dated, at a rate corresponding to the sampling rate of converters 72,82. This count made by the counter in the processor 89 is the number oferrors detected in demodulated signal from the receiver under test in atime period corresponding to the period between samplings taken byconverters 72, 82. The times of taking of signal samples from receivers70, 80 may coincide with updating of the output from the error detectoror may be at some other time within a sample period. The dataprocessorsystem 84 is designed to perform the following manipulations on datareceived

(1) to subtract one from the other digitised samples representing outputfrom the receivers 70 and 80 at the same time one from the other, toprovide in band amplitude dispersion (IBAD) values. The outputs of thereceivers are in decibel units so that this subtraction step isequivalent to division of the absolute (linear) magnitudes representedby these outputs.

(2) to provide a count of the numbers of occurrences of the differences,in decibels, generated in manipulation (1)over a range of suchdifferences

(3) to provide counts analogous to those accumulated in manipulation(2), but excluding from counting those differences which do not coincidewith conditioning of line 86 to states indicative that the error ratioin the demodulated data exceeds a predetermined value.

(4) to divide, for each difference, the counts obtained in manipulation(3) by the corresponding counts obtained in manipulation (2).

Referring now to FIG. 7, a radio receiver and demodulator under test areshown designated by reference numerals 100 and 102. The IF output fromthe receiver is shown connected to the line 98 providing input to theamplifiers 70 and 80 of the dynamic dispersion receiver 120. Line 98passes through receiver 120 to the demodulator 102. The line 86 to thesystem 84 is shown connected from the output of error detector 85associated with the demodulator 102. A digital data generator 106 isprovided generating a stream of data signals which may be of randomform. These are encoded by a transmitter 108 to form a digitallymodulated signal with the intermediate frequency of the receiver 100arranged as the carrier frequency. The signal is passed through asimulator 110, which may be of similar form to that described in FIG. 4and designed to provide at its output a combined signal simulating theeffect of multipath interference. With generator 106 operating, and withthe phase shift device 58 and attenuator device 56 of the simulator 110manipulated to constantly change the phase shift and amplitude providedthereby, the dynamic dispersion receiver 120 generates and processesdata as above described. In particular, the in band amplitude dispersionis computed at time spaced intervals and counts thereof for variousvalues are assembled. The plotter 92 is arranged to provide a directoutput generating a histogram of these counts and such a histogram isshown in FIG. 10. Since the resolution of the receivers 70, 80 is onehalf of a decibel, differences between these outputs are representableby one-half decibel figures and the plotter 92 is in this instancearranged to display accumulated numbers of occurrences of IBAD for eachIBAD value in the range minus 20 to plus 20 dB. The histogram, in thisinstance, shows a peak for number of occurrences at around the "0"decibel value for IBAD with a rapid falloff with increasing in bandamplitude distortion to either side. The plotter 92 directly plots thehistogram of the number of occurrences of different values of IBADcoincident with occurrence of detected error ratio in the output of thedemodulator being above a predetermined level such as 1 per one thousanddata bits. Such a histogram, which shows the results of the abovedescribed manipulation (2), is shown in FIG. 11, again with one halfdecibel resolution over the range minus 20to plus 20 dB IBAD. Thishistogram exhibits a typical configuration, having peaks to either sideof the "0" decibel position. Finally, the plotter 92 produces a plotsuch as shown in FIG. 12 generated by manipulation (3) above described,more particularly representing the probability that, for a given valueof in band amplitude dispersion, the given error ratio indicated abovewill be exceeded. The data processor system 84 may be programmed toproduce the necessary difference counts by subtraction but commerciallyavailable plotters may have the facility to automatically produce suchdifference counts.

The described histograms may be generated largely automatically by thereceiver 120 and plotter 92. Thus, the phase shifting device 58 and theattenuator device 56 may be arranged to automatically scan a range ofphase shifts and attenuations corresponding to moving of a responsenotch across the bandwidth of the signal being processed. This may beeffected by, for example, motorising the device or, if desired, byelectronic means. The histograms shown in FIGS. 10 and 11 are ofinterest in themselves. The histogram of FIG. 10 is useful in assessingthe overall quality of the input signal to the receiver under test. Thehistogram of FIG. 11 shows the combined effects of quality of the inputsignal and the quality of the demodulated signal.

Finally, the histogram of FIG. 12 demonstrates the quality of thereceiver and demodulator. Generally speaking, the larger the areaenclosed with the somewhat "U" shaped probability curve of thishistogram is indicative of the quality of the receiver and demodulator.FIG. 13 illustrates the effect of providing improved demodulating meansin the receiver, whereby the configuration of the probability curve isaltered to broaden it. The information provided by these histogramsmakes it possible to ascertain features of the performance of thereceiver and demodulator which might otherwise not be noted. This isillustrated in FIG. 13 in that the probability curve, whilst beinggenerally much better configured than that shown in FIG. 12, showsparticular peaks which illustrate relatively uneven performance atparticular IBAD values. These histograms too, incorporate data obtainedin a dynamic situation under changing conditions of the phase differenceprovided by the device 58 and attenuation device 56 and are thus morerepresentative of actual infield performance.

The dynamic dispersion receiver of the invention is usable incircumstances otherwise than merely for testing of receivers in alaboratory environment. FIG. 8 shows interconnections of componentsgenerally similar to those shown in FIG. 7, for infield testing. Heredata generator 106 is connected to an on-site transmitter 108 fordirection of signals via an antenna 109 to a receiving antenna 111.Thence, the signal is passed through the simulator 110 to the receiver100. The intermediate frequency output from the receiver 100 is thenpassed to the associated demodulator 102 and error detector 85, theintermediate frequency signal itself and the error detection outputsbeing passed to the dynamic dispersion receiver 120 as in the case ofFIG. 7.

Also, as shown in FIG. 9, the dynamic dispersion receiver of theinvention may be employed for in-field diagnosis of digital radio systemproblems. Here, the receiver 120 is shown connected to the output of theintermediate frequency section of a radio receiver 100 and alsoconnected on line 86 to receive the error output from a demodulator 102.Here, the simulator 110 is not provided, the receiver 100 simplyreceiving at its receiving antenna 111 signal directed from thetransmitter 106 via its antenna 109. In this instance, the system mayoperate to test normal digital traffic signal when multipath fading isoccurring as illustrated in FIG. 1.

FIG. 14 shows the arrangement of the receivers 70, 80 in greater detail.The receivers are of generally like form, each having a branchingamplifier 130 connected to the line 98 and designed to enable feed-offof signal from line 98 without interference with presentation of thesignal to the decoding device of the receiver under test. Output fromthe branching amplifiers as fed to respective mixers 132. The mixers 132receive oscillatory signals from respective local oscillators 134, 136.Signal from the mixers 132 is passed to respective amplifiers 138,thence through respective band pass filters 140, through respectiveamplifiers 144 to respective detectors 146.

The circuit details for the receivers 70, 80 are substantially the same,being in accordance with the representative circuit for the receiver 70shown in FIGS. 15, 16 and 17. FIG. 15 shows the mixer 132 receivinginput on a line 130b from the respective branching amplifier 130 and aline 134b from oscillator 134. The mixed signal is passed via acapacitor C3 to an amplifier device 138a which, together with resistorR1 and capacitor C4, comprises the amplifier 138 of receiver 70.Resistor RI and capacitor C4 are connected in parallel across thenegative supply terminal for the amplifier device 138. Output from theamplifier device 138 is taken directly to the filter 140 which comprisesa resistor R3 connected across the output of the amplifier device 138a,a series chain of components comprising resistor R2 and capacitors C5,C7, C9, C11 and C12 connected in that order between resistor R2 and theinput of a semiconductor amplifier device 144a which forms part ofamplifier 144 of receiver 70. Filter 140 also includes a capacitor C6and inductance L1 connected from the junction of capacitors C5 and C7 toground, a capacitor C8 and inductance L2 connected in parallel from thejunction of capacitor C7 and capacitor C9 to ground, a capacitor C10 andan inductance L13 connected from the junction between capacitors C9 andC11 to ground, a resistor R4 connected from the junction of capacitorsC11 and C12 to ground and a resistor C5 connected from the input ofdevice 144a to ground. In addition to the amplifier device 144aamplifier 144 includes the capacitor C13 shown connecting amplifierdevice 144a to ground, a resistor R6 connecting the output of device144a to ground, a series connected capacitor C14 and a resistor R7connected from the output of device 144a to the emitter of a transistorV1 also forming part of amplifier 144. Transistor V1 has its emitterconnected to ground via a resistor R8 and its base connected to groundvia a resistor R9 and parallel capacitor C6. The base of the transistorV1 is also connected to positive supply via a resistor R10. Amplifier144 is thus of conventional form, the device 144a providing for signalamplification and the transistor V1 and associated circuitry serving asa buffer amplifier. Output from the transistor V1 is taken via a ferritebead 141 to the primary winding of a transformer T1, the primary windingalso being connected to positive supply and to ground via capacitor C17.The secondary winding of the transformer T1 has a tuning capacitor C19connected thereacross and output from the secondary winding is taken tothe detector 146 which in this instance comprises a full wave bridgerectifier formed of four diodes D1, D2, D3, D4. Output from the detectoris taken via a resistor R11, a capacitor C20 being connected from theoutput side of resistor R11 to ground.

The local oscillators 134, 136 are of generally similar form althoughtuned to different local oscillator frequencies. Each may be of the formshown in FIG. 16 for the local oscillator 134. Here, the localoscillator 134 is shown as comprising a semiconductor oscillator device134a having its output connected via a matching pad comprised of threeresistors R12, R13, R14 to a line 134b which provides connection to themixer 132.

The branching amplifiers 130 for each receiver 70, 80 are likewise ofsimilar form and may be as shown in FIG. 17, more particularlycomprising a semiconductor amplifier device 130a having its inputconnected to line 98 via a resistor R15. The input to the device 130a isconnected to ground via a resistor R16. The output from device 130a istaken from a matching pad comprised of resistors R17, R18, R19 to a line130b providing connection to the mixer 132.

In an experimental device constructed in accordance with the invention,the mixer 132, amplifier device 138a, amplifier device 144a, transistorV1, oscillator device 134a and amplifier device 130 were commerciallyavailable components comprised as follows:

mixer 132: device type MCLSRA-1

amplifier device 138: SL5600

amplifier device 144a: SL5600

transistor V1: 2N5223

oscillator device 134a: Q023

amplifier device 130a: device type WJA74.

The components for the filters 140 are selected, in accordance withusual practice, to provide the necessary band pass characteristics todiscriminate against signals of frequency not corresponding to the mixedsignal components from the mixers 132 which relate to the desired narrowband frequencies for operation of the amplifiers 70, 80. In a practicalembodiment constructed in accordance with the invention, and intendedfor use with a 16 QAM 140 Mbit/s system utilizing a 70 MHz intermediatefrequency with a bandwidth of approximately 35 MHz it was foundsatisfactory to arrange the filters 140 to be tuned to a frequency of29.8 MHz with a bandwidth of approximately 500 KHz. In this case, localoscillators 134, 136 operated at 25.2 MHz and 114.8 MHz. This provided,for amplifier 70, a selection of a 500 KHz wide segment of the totalbandwidth of the incoming signal, centered on 55 MHz (i.e. towards thelowest end of the bandwidth) and for amplifier 80 a 500 KHz segmentlocated at 85 MHz (i.e. adjacent the upper range of the bandwidth of theincoming signal).

FIGS. 18 and 19 show data processing steps for producing data for makingthe plots such as those shown in FIGS. 10, 11, 12 and 13. This flowchart has been found suitable for implementation with a commercialdataprocessor Honeywell type H6000. The implementation enables dataprocessing, where the data represented by output from the converters 72and 82 is presented indirectly to the dataprocessor system 84 of thereceiver 120, having been pre-recorded on tape. The system is designedfor implementation where the data is so-presented is arranged in thetape with blocks of data representing samples taken at ten per secondand recorded as two consecutive sub-blocks each having data for twoconsecutive five second data periods. In the first step, indicated at150, the data is taken from the storage tape to a disc storageassociated with the dataprocessor 84. In a second step, labelled 152,the first five second sub-block of data in one of the aforementioned tensecond full data blocks is unpacked In a third step indicated at 154,the signal levels from the analog to digital converters 72, 82 areconverted to decibel indications. In this regard, there is, generally, anon-linear relationship between the voltage delivered by the receivers70 and 80 and the actual decibel levels indicated thereby and the step154 is therefore necessary to convert the signal levels to decibels. Ata step 156, the difference between the two decibel levels determined instep 154 is determined to give the IBAD values. In a step 158 these IBADvalues are stored in a fashion enabling them to be later read outtogether with information indicating the time at which the IBAD valueswere determined in the five second data capture period. Next, at a step160, memory locations in a random address memory are incrementeddepending on the IBAD values stored at step 158. Thus, for eachoccurrence of a particular IBAD value, within the fifty sample period, acorresponding location for accumulation of numbers of occurrences ofthat IBAD value is incremented by one unit. At a step 170, the number ofpoints in the data period is counted and when the full number for thefive second period has been fulfilled, the program moves to step 172 asshown where, stored data transferred from the aforementioned tape to thedisc at step 150 and detailing the bit error ratio as supplied on line86 is unpacked. Then, in a step 174, the accumulated bit error ratiosfor consecutive sample periods within the five second data period arecomputed by accumulation of bit error counts within each such period.Next, in a step 176, the so determined bit error ratios are comparedwith a threshold value representing the desired bit error ratio to beused in forming the histogram of FIG. 11. If these are determined not tobe greater than this threshold, this information is transferred for usein a subsequent step 180. If they are determined to be greater,incrementation of a memory array location is effected in correspondencewith the corresponding IBAD value relating to the time period for whichthat bit error ratio was computed. This is effected in conjunction withinformation stored in step 158. That is to say, if a Particularly storedIBAD value is found to occur when the bit error ratio at a correspondingsample time was greater than the threshold established in step 176, anarray location, for storage of accumulated counts of occurrence of thatIBAD value under the condition that the error threshold is exceeded, isincremented at step 178. At a step 180, a determination is made as towhen the whole of the five second data sub-pack has been processed. Oncompletion of the five second data sub-pack, as determined at step 180,the program starting from step 154 is repeated for the second fivesecond data period. At a step 182, a determination is made as to when afull ten second data period has been processed and if so determinationis then made at a step 186 as to whether all of the data to be processedhas been so processed. If not all data has been processed, the programis begun again from step 152 by unpacking of the first five second dataperiod for the next ten seconds of data. In the event that all data isdetermined as being finished at step 186, the program proceeds to step188 to output data to a disc file and thence to proceed to stop theprogram at step 190.

In this specification, including the appended claims, references tomathematical operations such as division or multiplication, wherequantities are expressed in ordinary "linear" units, are to beunderstood as including references to corresponding mathematicallyequivalent operations where the quantities are expressed in other units.For example, operations where quantities are expressed in linear units,and involving division or multiplication, are equivalent to operationsinvolving subtracting or adding the equivalent logarithmic (e.g.decibels) values of those quantities. Conversely, references tomathematical operations specified in relation to quantities expressed indecibels are to be taken as references to mathematically equivalentoperations on the quantities when expressed in linear units. In thedescribed embodiments, the accumulated counts of the number ofoccurrences of IBADs of particular values, and the accumulated counts ofoccurrences of IBADs of particular values which correspond with themeasured error ratio crossing a predetermined level, are divided oneinto the other. This count division is to be taken as implying divisionof absolute or linear values of the counts or as implying any equivalentmathematical operation. For example, the counts could themselves beexpressed in decibels or other logarithm units, such that the divisionwas effected by subtraction of those so expressed counts. Thusreferences in this specification, including the appended claims, to"division" of these counts are to be taken as encompassing division byany mathematically equivalent process.

While the invention has been specifically described in relation tocharacterisation of digitally modulated signals, the invention isequally applicable to characterisation of analogue modulated signals. Inthis case, instead of monitoring the error ratio as described anothersuitable parameter describing the signal quality of the demodulatedcarrier signal may be monitored. For example the signal to noise ratioof the demodulated signal may be so monitored. Then, the method of theinvention may be practised by accumulating numbers of occurrences ofIBADs of respective different values, and the numbers of occurrences ofrespective IBADs of different values and which at least coincide withthe value of the relevant parameter, such as signal to noise ratio,crossing a predetermined level. Divided counts obtained by division ofthe second of these counts by the first mentioned are then obtained inan analogous way to that described in relation to digital signalcharacterisation.

A program listing for execution of the program described in FIGS. 18 and19 follows: ##SPC1##

This program permits processing of two sets of data obtained from twoseparate amplifier dispersion receivers 120 and also permitsaccumulation of, for each such data set, assembly of data for productionof six histograms of the form shown in FIG. 11 from data from sixdifferent types of demodulator.

The plotting of histograms of FIGS. 10, 11 and 12 is effected from thedata stored at steps 158 and 178 in the flow chart of FIG. 18. Programsof well known form may be used to divide, for each IBAD value, thestored counts in the arrays of such counts stored at steps 158 and 178,for the purpose of generating values for plotting the histogram of FIG.12.

Although the described implementation operates for pre-recorded data thedata processing program may be readily adapted to operate on real-timedata with a dedicated processor system, such as Motorola type M68000.

The described method and apparatus generates a plot such as shown inFIG. 12 representing the probability that, for a given value of in bandamplitude dispersion, the given error ratio indicated above will beexceeded. The method and apparatus may however be modified forgeneration of an "inverse" plot representing the probability that, for agiven value of in band amplitude dispersion, a given error ratio willnot be reached. In this instance, the plotter 92 may simply be set toplot the histogram of the number of occurrences of different values ofIBAD coincident with occurrence of detected error ratio in the output ofthe amplifier being below a predetermined level such as 1 per onethousand data bits.

The invention has been described merely by way of example only and manymodifications may be made thereto without departing from the spirit andscope of the invention as defined in the appended claims

I claim:
 1. A method of visually characterising a modulated carriersignal comprising the steps of:repetitively sampling the level and signof the amplitude dispersion of the signal during a test period, dividingthe range of possible amplitude dispersion levels, both positive andnegative, into a series of sub-ranges, counting the number of times thatthe sampled level falls within each of said sub-ranges therebyaccumulating a first count for each sub-range, and generating agraphical representation of count against sub-range therein first countsfor sub-ranges in which the amplitude dispersion is positive aredisplayed together with first counts for sub-ranges in which theamplitude dispersions is negative so that differences therebetween willbe apparent.
 2. A method according to claim 1, including the stepexaggerating said differences by increasing the counts accumulated forthose sub-ranges encompassing the higher levels of amplitude dispersionrelates to the counts accumulated for those sub ranges encompassing thelower levels of amplitude dispersion.
 3. A method according to claim 1including the step of exaggerating said differences by arranging thatthe sub-ranges which encompass higher levels of sampled amplitudedispersion of one sign overlap those which encompass lower levels ofsampled amplitude dispersions of the same sign.
 4. A method according toclaim 1 including the steps of exaggerating said differences byarranging the sub-ranges to be non-overlapping and contiguous, andgenerating the count corresponding to each sub-range employed in saidgraphical representation by adding the sum of the first count for thatsub-range (if any) encompassing a lower level of amplitude dispersion ofthe same sign.
 5. A method according to any one of claims 1-4 includingthe steps of:monitoring a parameter indicative of the quality of themodulated carrier signal, and including only that sub-set of said firstcounts in said graphical representation for which said parameter lieswithin predetermined limits.
 6. A method according to any one of claims1-4 including the steps of:monitoring a parameter indicative of thequality of the modulated carrier signal, counting the number of timesthe sampled amplitude dispersion of the signal falls within eachsub-range when said parameter lies within predetermined limits therebyaccumulating second count for each sub-range, determining the ratio ofsaid second and first counts for each sub-range, and generating saidgraphical representation by displaying said ratio against sub-rangeinstead of said first count against sub-range.
 7. A method ofcharacterising the performance of a digital microwave receiver connectedto receive and demodulate a digitally modulated microwave carrier signalwhich may be subject to distortion due to multipath fading, said methodcomprising the steps of:repetitively measuring, at time spaced intervalsduring a test period, the level and sign of the amplitude dispersion ofthe signal as received. detecting and monitoring the error rate in thesignal as received demodulated by the receiver, accumulating firstcounts of the number of times that said dispersion measurements fallwithin each one of a series of ranges of possible dispersion levels,both positive and negative, when said error rate lies withinpredetermined limits, and generating a graphical representation of countagainst range wherein said first counts for ranges in which theamplitude dispersion is positive are displayed together with those forwhich the amplitude dispersion is negative so that any differencestherebetween will be apparent.
 8. A method of characterising theperformance of a digital microwave receiver connected to receive anddemodulate a digitally modulated microwave carrier signal which may besubject to distortion due to multipath fading, said method comprisingthe steps of:detecting and monitoring the error rate in the signal asreceived and demodulated by the receiver under test, when said errorrate lies between predetermined limits, repetitively measuring attime-spaced intervals during a test period the level and sign of theamplitude dispersion of the signal as received, and accumulating firstcounts of the number of times that said dispersion measurement fallwithin each one of a series of ranges of possible dispersion levels,both positive and negative, and displaying said first count againstmeasurement range for both positive and negative dispersion levels.
 9. Amethod according to either claim 7 or 8 including the stepsof:accumulating total counts of the number of dispersion measurementfalling within each of said ranges without regard to said error rate,determining the ratio of said first counts to said total counts for eachrange where there are first counts, and generating a graphicalrepresentation by displaying said ratio against range instead of saidfirst count against range.
 10. Electronic test equipment forcharacterising a modulated carrier signal during a test period, saidequipment comprising:measuring means for measuring the amplitudedispersion of the signal and for generating, at time-spaced intervals,outputs having values indicative of the magnitude and sign of theamplitude dispersion at the respective intervals, classifier means,connected to receive said outputs of said measuring means, forgenerating in respect of each of said outputs a range signal indicatingin which one of a series of possible ranges of said output values thatoutput value falls, first counter means, connected to receive said rangesignals from said classified means, for accumulating first points of thenumber of times each range is indicated, and read-out means for readingout at the end of the test period the first counts accumulated, range byrange, so that a graphical display device can generate a display offirst count against range.
 11. Electronic test equipment according toclaim 10 where, in addition to the modulated carrier signal, a secondsignal indicative of the instantaneous quality of the carrier signal isavailable, said test equipment comprising:monitoring means for receivingsaid second signal and for generating an output signal whenever thesecond signal lies within a predetermined range, count control meansconnected between said monitoring means and said first counter means forinhibiting the accumulation of said first counts when said output signalis generated by said monitoring means.
 12. Electronic equipmentaccording to claim 11 comprising:second counter means, connected toreceive said range signals from said classifier means, for accumulatingsecond counts of the number of times each range is indicated, anddivider means connected to said first and said second counter means forgenerating an output indicative of the ratio of said first and saidsecond counts accumulated in respect of each range, and said readoutmeans includes selector means connected to select either accumulatedcount or said ratio for read-out to the graphical display device.
 13. Aninstrument for indicating the performance of a digital microwavereceiver when the received signal may be subject to multipath fading andan error signal indicative of the instantaneous error rate in thedemodulated output of the receiver is available, said instrumentcomprising:measuring means for measuring the amplitude dispersion of thereceived signal and for generating, at time-spaced intervals, outputshaving values indicative of the magnitude and sign of the amplitudedispersion at the respective intervals, classifier means, connected toreceive said outputs of said measuring means, for generating in respectof each of said outputs a range signal indicating in which one of aseries of possible ranges of said output values that output value falls,first counter means, connected to receive said range signals from saidclassifier means, for accumulating first counts of the number of timeseach range is indicated, and graphical display means connected to saidfirst counter means for generating a graphical representation of countagainst range wherein said first counts for ranges in which theamplitude dispersion is positive are displayed together with those forwhich the amplitude dispersion is negative so that any differencestherebetween will be apparent.
 14. An instrument according to claim 13comprising:monitoring means for receiving the error signal and forgenerating an output signal whenever the corresponding error rate lieswithin a predetermined range, count control means connected between saidmonitoring means and said first counter means for inhibiting theaccumulation of said first counts when said output signal is generatedby said monitoring means.
 15. An instrument according to claim 14comprising:second counter means, connected to receive said range signalsfrom said classifier means, for accumulating second counts of the numberof times each range is indicated, and selector means connected to selectbetween the accumulated first count, the accumulated second count andsaid ratio of said counts for transmission to said graphical displaymeans, range by range.
 16. An instrument according to claim 14comprising:second counter means, connected to receive said range signalfrom said classifier means, for accumulating second counts of the numberof times each range is indicated, and divider means connected to saidfirst and said second counter means for generating an output indicativeof the ratio of said first and said second counts accumulated in respectof each range, and selector means connected to select between theaccumulated first count, the accumulated second count and said ratio ofsaid counts for transmission to said graphical display mans, range byrange.